Why did Jesus do what must be considered "not of this world" supernatural signs, or miracles? Why couldn't He have merely been a normal rabbi teaching do-good morals of Judaic law and interpretation - and left it at that?

Essentially, there are three reasons.

First, to show people that He and the Creator God were one and the same.

Second, to cause His disciples to believe that He and the Creator God were one and the same, per:

John 2:11 This, the first of His signs, Jesus did at Cana in Galilee, and manifested His glory; and His disciples believed in Him.

Third, simply because He could.

And why did He do what He could?

Because He wanted to do what He could.

End of line.

But there's more.

He wanted to promote heterosexual marriages instead of homosodomy-unions-licensing "celebrations," and facilitate the drinking of that potentially-intoxicating alcoholic beverage [of whatever alcohol percentage] called "wine."

One baptist [small "b"] preacher has blattered that people of those days had to drink wine, because the water was lousy, and he presumed that the water mentioned in John's gospel pertaining to the "wedding" [not marriage?] celebration in Cana in filled waterpots of 20-30-gallon capacity (like the size of a large trash bag in a big garbage can) contained dirty water which had been used for dishwater and footwashing, but which Jesus turned into non-alcoholic "grape juice."

What the baptist preacher said sounds polluted - all the way around. Hard to say where the baptist preacher came up with such weird ideas and non-authorized additions to Scriptural Text. Perhaps he craved being different to be reputed as sensational. Does that not endanger his job as both senior pastor and head of the baptist seminary?

Actually, guessing how particular genuine Jews required kosher purity, the water pots mentioned in:

John 2:6 Now six stone jars were standing there, for the Jewish rites of purification, each holding twenty or thirty gallons. ....probably contained at least the equivalent of non-dirty water, though perhaps not of the rather-pure Ice Mountain or Fina or other bottled spring water quality - but logically not filthy waste water from the toilet or from foot-washing (though it would have become waste water after used then discarded for footwashing).

It does not make sense that Jews would wash their hands and feet with already-dirty water, does it?

Now, for the alleged "grape juice."

It seems that the aforementioned Baptist bellerer presumed that knowledgeable-astute-and-learned Greek with their advanced-enough Greek vocabulary could not semantically distinguish between the two totally-different entities of [non-fermented] "grape juice" contrasted with [fermented] "wine."

The word for "wine" which the steward tasted and used in the John chapter 2 passage:

John 2:1 On the third day there was a marriage [not "wedding"] at Cana in Galilee, and the mother of Jesus was there;
2:2 Jesus also was invited to the marriage, with His disciples.
2:3 When the wine failed [ran out, was depleted?], the mother of Jesus said to Him: "They have no wine [Gr. oinos]."
2:4 And Jesus said to her, "Oh woman [Gr. gyn(e)], what have you to do with me? My hour has not yet come."
2:5 His mother said to the servants, "Do whatever He tells you."
2:6 Now six stone jars were standing there, for the Jewish rites of purification, each holding twenty or thirty gallons.
2:7 Jesus said to them, "Fill the jars with water [not necessarily toilet or footwashed water]." And they filled them up to the brim.
2:8 He said to them, "Now draw some out, and take it to the steward of the feast." So they took it.
2:9 When the steward of the feast tasted the water now become wine, and did not know where it came from (though the servants who had drawn the water knew), the steward of the feast called the bridegroom
2:10 and said to him, "Every man serves the good [kosher] wine first; and when men have drunk freely, then the poor wine; but you have kept the good wine until now."
2:11 This, the first of His signs, Jesus did at Cana in Galilee, and manifested His glory; and His disciples believed in Him.

"Just gimme some kind of sign, baby, oh my darlin'. Show me that your mine, girl, oh yeah."

[ Sorry about that. Reverting to pop-rock music. It happens. ]

....means wine and wine only - not [non-fermented] grape juice!

The word wine [not "grape juice"] throughout the entire Bible is meant to accomplish several things for or against human beings:

Ruth 3:7 And when Boaz had eaten and drunk, and his heart was merry [not "drunk"], he went to lie down at the end of the heap of grain. Then she came softly, and uncovered his feet, and lay down.

First Samuel 25:36 [GWV] When Abigail came to Nabal, he was holding a banquet in his home. It was like a king’s banquet. He was in a good mood [not "merry"] and very drunk, so she didn’t tell him anything until dawn.
36 [Message] When Abigail got home she found Nabal presiding over a huge banquet. He was in high spirits [not "merry"] and very, very drunk. So she didn’t tell him anything of what she’d done until morning.
36 [Rotherham] And, when Abigail came unto Nabal, lo! he, had a banquet in his house, like the banquet of a king, and, the heart of Nabal, was glad [not "merry"] accordingly, he having drunk deeply, so she told him nothing less or more, until the light of the morning.
36 [YLT] And Abigail cometh in unto Nabal, and lo, he hath a banquet in his house, like a banquet of the king, and the heart of Nabal is glad [not "merry"] within him, and he is drunk unto excess, and she hath not declared to him anything, less or more, till the light of the morning.

Esther 1:10 On the seventh day, when the heart of the king was merry [not "drunk"] with wine, he commanded Mehuman, Biztha, Harbona, Bigtha and Abagtha, Zethar and Carkas, the seven eunuchs who served King Ahasuerus as chamberlains [to bring pretty-but-feminist-sexist-insubordinate Queen Vashti]...

Job 12:25 They grope in the dark without light; and he makes them stagger like a drunken man.

Psalm 104:14 You do cause the grass to grow for the cattle, and plants for man to cultivate, that he may bring out food from the earth, Psalm 104:15 and wine to gladden the heart of humans, oil to make his/her face shine, and bread to strengthen man's/woman's heart.

Ecclesiastes 9:7 Go, eat your [intoxicating?] bread with enjoyment, and drink your wine with a merry heart; for God has already approved what you do.

Isaiah 19:14 The LORD has mingled within her a spirit of confusion; and they have made Egypt stagger in all her doings as a drunken man staggers in his vomit.
Isaiah 28:7 These also reel with wine and stagger with strong drink; the priest and the prophet reel with strong drink, they are confused with wine, they stagger with strong drink; they err in vision, they stumble in giving judgment.
Isaiah 29:9 Stupefy yourselves and be in a stupor, blind yourselves and be blind! Be drunk, but not with wine; stagger, but not with strong drink!

Jeremiah 25:16 They shall drink and stagger and be crazed because of the sword which I am sending among them."

Obadiah 1:16 For as you have drunk upon my holy mountain, all the nations round about shall drink; they shall drink, and stagger, and shall be as though they had not been.

Habakkuk 2:16 You will be sated with contempt instead of glory. Drink, yourself, and stagger! The cup in the LORD's right hand will come around to you, and shame will come upon your glory!

Clearly, drunkenness and wine do not necessarily go hand in hand:

Ephesians 5:18 And do not get drunk with wine, for that is debauchery; but be filled with the Spirit...

First Timothy 3:8 Deacons likewise must be serious, not double-tongued, not addicted to much wine, not greedy for gain;
First Timothy 5:23 No longer drink only water, but use a little wine for the sake of your stomach and your frequent ailments.

Revelation 6:6 and I heard what seemed to be a voice in the midst of the four living creatures saying, "A quart of wheat for a denarius, and three quarts of barley for a denarius; but do not harm oil and wine!"

One obviously cannot get drunk on [non-fermented] grape juice! Nor does one have to drink wine in excess to the extent that:

Proverbs 23:29 Who has woe? Who has sorrow? Who has strife? Who has complaining? Who has wounds without cause? Who has redness of eyes?
23:30 Those who delay long over wine, those who go to try mixed wine.
23:31 Do not look at wine when it is red, when it sparkles in the cup and goes down smoothly.
23:32 At the last it [grape juice?] bites like a serpent, and stings like an adder.
23:33 Your eyes will see strange things, and your mind utter perverse things.
23:34 You will be like one who lies down in the midst of the sea, like one who lies on the top of a mast.
23:35 "They struck me," you will say, "but I was not hurt; they beat me, but I did not feel it. When shall I awake? I will seek another drink [of water? grape juice? milk?]."

The excuse of some that if someone tastes wine for the very first time, they automatically become addicted-to-it drunkards, is not realistic, although:

Genesis 9:20 Noah was the first tiller of the soil. He planted a vineyard;
Genesis 9:21 and he drank of the wine, and became drunk, and lay uncovered in his tent.

If one plays with matches, they automatically become an arsonist? If one smells glue, they automatically become an addicted glue-sniffer? If one gets firearms training, they automatically become a genocidal sniper? If one picks up a cigarette butt off the ground to throw in the trash, they automatically become an addicted chain smoker? If one eats too many prune, they automatically get diarrhea?

A paraphrase of the remark that Jesus made to His human mother was noteworthy:

"Lady [or ma'am, or woman] what are you trying to get me to do, and for what purpose?"

What is for the same proud promotional purpose as:

Matthew 20:20 Then the mother of the sons of Zebedee came up to him, with her sons, and kneeling before him she asked him for something.
20:21 And he said to her, "What do you want?" She said to him, "Command that these two sons of mine may sit, one at your right hand and one at your left, in your kingdom."
20:22 But Jesus answered, "You do not know what you are asking. Are you able to drink the cup that I am to drink?" They said to him, "We are able."
20:23 He said to them, "You will drink my cup, but to sit at my right hand and at my left is not mine to grant, but it is for those for whom it has been prepared by my Father."
20:24 And when the ten heard it, they were indignant at the two brothers.

Why the ten were indignant at the two brothers is perplexing, being that they should instead have been indignant against the mother of the two sons. Perhaps feminism came into play even back then, but perhaps it did not - if the two brothers got their mom to make such a request to Jesus.

Matthew 20:25 But Jesus called them to Him and said, "You know that the rulers of the Gentiles lord it over them, and their great men exercise authority over them.
20:26 It shall not be so among you; but whoever would be great among you must be your servant,
20:27 and whoever would be first among you must be your slave;
20:28 even as the Son of Man came not to be served but to serve, and to give His life as a ransom for many."

Mother Mary's petition to Jesus to make wine, or go and buy some, or whatever reminds me of another previous time when someone petitioned the Lord to "show Himself to prove who He was" for the sake or entertainment of whoever:

Matthew 4:1 Then Jesus was led up by the Spirit into the wilderness to be tempted by The Devil.
4:2 And He fasted forty days and forty nights, and afterward He was hungry.
4:3 And The Tempter came and said to Him, "IF (not "since" - contrary to what pseudo-"priest" Bart Ryan said) you are the Son of God, command these stones to become loaves of bread."
4:4 But He answered, "It is written, 'Man shall not live by bread alone, but by every word that proceeds from the mouth of God.'"
4:5 Then The Devil took him to the holy city, and set Him on the pinnacle of the temple,
4:6 and said to Him, "IF you are the Son of God, throw yourself down; for it is written, 'He will give His angels charge of you,' and 'On their hands they will bear you up, lest you strike your foot against a stone.'"
4:7 Jesus said to Him, "Again it is written, 'You shall not tempt the Lord your God.'"
4:8 Again, The Devil took him to a very high mountain, and showed Him all the kingdoms of the world and the glory of them;
4:9 and he said to Him, "All these I will give you, if you will fall down and worship me."
4:10 Then Jesus said to him, "Begone, Satan! for it is written, 'You shall worship the Lord your God and Him only shall you serve.'"

But did Jesus "tell Mary off" in a sassy rude way?

Not unheard of in the annals of Old-Testament history:

Hosea 4:5 You shall stumble by day, the prophet also shall stumble with you by night; and I will destroy your mother.

....and what was to come in the future:

Matthew 12:48 And He answered and said to him who told Him: "Who is My mother? And who are My brothers?"
Matthew 12:49 And He stretched out His hand toward His disciples and said: "Hey, My mother and My brothers!
Matthew 12:50 For whoever shall do the will of My Father in Heaven, the same is My brother and sister and mother."

Mark 3:32 And a crowd was sitting around Him; and they said to Him: "Your mother and your brothers are outside, asking for you."
Mark 3:33 And He replied: "Who are my mother and my brothers?"
Mark 3:34 And looking around on those who sat around Him, He said: "Here are my mother and my brothers!
Mark 3:35 Whoever does the will of God is my brother, and sister, and mother."

So much for Catholic or Protestant family loyalty, when family loyal comes into conflict against allegiance to the Holy Father (and you know Who I non-blasphemously refer with the phrase: "Holy Father" - yes?).

Luke 14:26 "If any one comes to me and does not hate his own father and mother and wife and children and brothers and sisters, yes, and even his own life, he cannot be my disciple."

Exodus 20:12 "Honor your father and your mother, that your days may be long in the land which the LORD your God gives you."
Deuteronomy 5:16 "Honor your father and your mother, as the LORD your God commanded you; that your days may be prolonged, and that it may go well with you, in the land which the LORD your God gives you."
Matthew 15:4 For God commanded, 'Honor your father and your mother,' and, 'He who speaks evil of father or mother, let him surely die.'
Matthew 19:19 "Honor your father and mother, and, You shall love your neighbor as yourself."
Mark 7:10 For Moses said: 'Honor your father and your mother'; and, 'He who speaks evil of father or mother, let him surely die';
Mark 10:19 You know the commandments: 'Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and mother.'"
Luke 18:20 You know the commandments: 'Do not commit adultery, Do not kill, Do not steal, Do not bear false witness, Honor your father and mother.'"
Ephesians 6:2 "Honor your father and mother" (this is the first commandment with a promise)...

Mother Mary was, by that time, ready to not respond critically back to her Son Jesus, because He had previously said something similar along those lines indicating that He was more than a mere human son:

Luke 2:41 Now His parents went to Jerusalem every year at the feast of the Passover.
2:42 And when He was twelve years old, they went up according to custom;
2:43 and when the feast was ended, as they were returning, the boy Jesus stayed behind in Jerusalem. His parents did not know it,
looked for Him among their relatives and acquaintances;
2:45 and when they did not find Him, they returned to Jerusalem, seeking Him.
2:46 After three days they found Him in the temple, sitting among the teachers, listening to them and asking them questions;
2:47 and all who heard him were amazed at His understanding and His answers.
2:48 And when they saw Him they were astonished; and his mother said to Him: "Son, why have you treated us so? Hey, your father and I have been anxiously looking for you."
2:49 And He said to them: "How is it that you sought me? Did you not know that I must be in my Father's house?"
2:50 And they did not understand the saying which He spoke to them.
2:51 And He went down with them and came to Nazareth, and was obedient to them; and His mother kept all these things in her heart.

So, the blessed post-virgin, who finally genitalized Joseph to get more progenie, and upon hearing Christ's "Oh woman, it's beyond your pay-grade to ask me that" remark, did not retort with:

"Now look here, Kid. You're not curtseying to me with the worship I deserve as Your Immaculate Holy Mary, Mother of Gawd, Co-Redemptrix who You and a lot of other goofs will be praying to later on."

Thus, Jesus did not have to counterattack, blattering:

"Go, stink up the place with some voodoo incense, indulge yourself sensuously fondling your beads, then screw yourself after your demise in Purgatory, you old relic."

Yet, Mary did not give up at the Cana marriage feast, because she was called by God to do what was to cause a miracle to happen, and apparently she had the clout to aggressively advise some guys or gals in high places to accommodate to whatever Son Jesus might say.

And sure enough, Son Jesus - at the behest of His mom's suggestion [ "behind every great man is a great mom?" ], did think that now was the time to respond positively to His mother Mary (unlike when He had previously responded negatively in the desert to the Tempter).

Intriguing it is that Jesus did not lift a finger to help out. Nor did he run to the nearest liquor store to buy wine. He commissioned other guys to do the manual labor of filling waterpots, then become couriers to the steward of the feast.

Perhaps the couriers trusted Mary to do the dangerous (and perhaps ridiculous) thing of taking potted water in a drinking cup to the steward of the feast. Maybe they also trusted Jesus to not embarrass them by so doing.

Whatever the case, we scientists now know that energy and matter are interchangeable, according to Einstein's equation E=mc squared. But controlled and rather instantaneous transmutation of matter of the molecular composition of water to matter of the molecular composition of wine is currently way beyond our present technology, and there is no known way that Jesus did that using any scientific technique or apparatus but simply by ordering some of it transported to some steward of some marriage feast. If, then how, the remainder of the water in the six waterpots was assumed to also be transformed from water to wine is not known.

Such control of natural phenomena is not to be unexpected from the Creator Christ who was (and yet is!) one and the same with His Father and The Spirit:

John 1:1 In the beginning was the Word, and the Word was with God, and the Word was God.
John 1:2 He was in the beginning with God;
John 1:3 all things were made through Him, and without Him was not anything made that was made.
John 1:14 And the Word became flesh and dwelt among us, full of grace and truth; we have beheld His glory, glory of the only Son from the Father.

John 8:58 Jesus said to them, "Truly, truly, I say to you, before Abraham was, I AM."

John 10:30 I and the Father are one."

John 14:9 Jesus said to him, "Have I been with you so long, and yet you do not know me, Philip? He who has seen me has seen the Father; how can you say, 'Show us the Father'?

Colossians 1:9 And so, from the day we heard of it, we have not ceased to pray for you, asking that you may be filled with the knowledge of His will in all spiritual wisdom and understanding,
1:10 to lead a life worthy of the Lord, fully pleasing to Him, bearing fruit in every good work and increasing in the knowledge of God.
1:11 May you be strengthened with all power, according to His glorious might, for all endurance and patience with joy,
1:12 giving thanks to the Father, who has qualified us to share in the inheritance of the saints in light.
1:13 He has delivered us from the dominion of darkness and transferred us to the kingdom of his beloved Son,
1:14 in whom we have redemption, the forgiveness of sins.
1:15 He is the image of the invisible God, the first-born of all creation;
1:16 for in Him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or principalities or authorities....all things were created through Him and for Him.
1:17 He is before all things, and in Him all things hold together.

Hebrews 1:1 In many and various ways God spoke of old to our fathers by the prophets;
1:2 but in these last days He has spoken to us by His Son, whom He appointed the heir of all things, through whom also He created the world.
1:3 He is the brightness of God's glory and the express image of His person, upholding all things by the word of His power. When He had by Himself purged our sins, He sat down at the right hand of the Majesty on high,
1:7 Of the angels He says, "Who makes His angels winds, and His servants flames of fire."
1:8 But of the Son He says, "Your throne, God, is forever, the righteous scepter is the scepter of your kingdom.
1:9 You have loved righteousness and hated lawlessness; therefore God, your God, has anointed you with the oil of gladness beyond your comrades."
1:10 And: "You, Lord, did form Earth in the beginning, and the heavens are the work of your hands....

First John 5:7 For there are three who bear record in Heaven: the Father, the Word, and the Holy Spirit; and these three are one.